<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>Document</title>
    </head>
    <body>
        <section>
            <section>
                <h1>123</h1>
            </section>
        </section>
        <!-- section 下的 h1 font-size = 1rem 是浏览器自带的样式  em取值是父标签的font-size * 1 -->
    </body>
</html>
